1. Field of the Invention
The invention relates to an apparatus and method for stacking elongated articles in spaced layers. In particular, the invention relates to a push-on type apparatus and method for stacking lumber utilizing a rotary cam drive mechanism.
2. Description of the Prior Art
The lumber industry throughout the world has become highly mechanized in recent years to a point that the most time consuming operation presently is the stacking of lumber after it has been sawed at the sawmill. The term "stacking" refers to the laying of a predetermined number of boards of horizontal supports thus forming layers or courses of lumber. The time-honored manual method of stacking lumber has been to first form a bottom layer of boards upon a support and then to successively form additional layers which are spaced apart by stacking sticks, or check sticks as they are commonly called. The stacking sticks are manually placed across the top of each layer of boards with predetermined spaces between them, usually 2 by 4 feet. The manual placing of lumber and stacking sticks in alternating layers is continued until the package of lumber is at the desired height.
In recent years mechanical lumber stackers have gained acceptance in large sawmills. The prior art stackers generally operate as follows: Lumber is placed on a haul-up which delivers it to an unscrambler such as described in U.S. Pat. No. 3,204,753. The unscrambler untangles the lumber and conveys it up an incline where it delivers individual boards to a horizontal conveyor known as a course maker. A complete layer of boards is made up on this course maker. A mechanized set of arms rise under the layer of boards, pick it up and move it out over the package of lumber. These arms then lower to a point where a stop prevents the back edge of this layer of boards from returning to the course maker and the arms retract thereby depositing the layer of boards on the package. A lumber stacker of this general type is described in U.S. Pat. No. 3,743,113. In this type operation, the stacking sticks may be placed loosely in U-shaped holders on each side of the stack since the layer of boards is being placed onto the sticks and not slid over the sticks. Although the above-described lumber stackers have been available for several years and many have been in operation in high-production sawmills throughout the world, the price of these machines is extremely high thereby prohibiting their economical use in small and medium-sized sawmills. Furthermore, these arm-type stackers must be made extremely large and must support huge loads when stacking long boards, e.g., 30 to 40 feet. Therefore, it becomes an object of the present invention to provide a lumber stacking apparatus which may be economically produced and is suitable for small and medium-sized sawmills and which may be used to stack long boards as efficiently as short boards.
In regard to the cam drive mechanism of the present invention, it should be noted that it has been proposed to load ties utilizing a rotatable cam-type element which is specifically contoured to contact the side of the tie and impart a substantially uniform acceleration to the tie as it is loaded. Such a tie loader is described in U.S. Pat. No. 3,508,672. The cam disclosed in this patent is shaped somewhat like a "beaver tail" and is primarily adapted for imparting a constant acceleration to the tie. The force vector at the contact surface between the cam and the side of the tie is not at a constant angle with the vertical, but rather changes directions substantially during the momentary pushing operation. If such a cam shape were utilized to drive lumber onto a package, indications are that it would frequently cause the boards to rise over the cam or "buckle" due to the resistance caused by the lumber already on the package.